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A while back I discovered a fantastic article about time travel on Slate.com. The author, Dave Goldberg, both a physicist and self-proclaimed science-fiction geek, wrote the post back in 2009 in anticipation of the film adaptation of THE TIME TRAVELER’S WIFE by Audrey Niffenegger. In the article, Goldberg takes a broad look at time travel in contemporary books and films, from BACK TO THE FUTURE to LOST to THE TERMINATOR. Ultimately, Goldberg takes the position that, at least to a physicist, THE TIME TRAVELER’S WIFE gets the science the closest to what might be considered scientifically sound.

If you’re writing a time-travel story, I highly recommend a full read of Goldberg’s article. It can be found by clicking this link. Below, for a more general overview, I’ve shared the four rules Goldberg says are necessary for scientifically sound, fictional time travel.

1) This is the only universe you’ve got.

Most of us have heard of the major scientific breakthrough of the last century known as quantum physics. One researcher in quantum physics, Hugh Everett, took the theories of quantum mechanics and imagined that perhaps the universe was constantly multiplying. He proposed the theory that, as electrons and other particles moved, the universe made almost perfect copies of itself. Since electrons and the like move constantly, these universes would be constantly multiplying, with no limit to the universes created.

This “many universes” theory lends itself well to the type of time travel depicted in BACK TO THE FUTURE, where characters travel between times and different realities are possible. A change in behavior in the past can lead to a different future. But according to Goldberg, this type of theory is at odds with Einstein’s Theory of Relativity, the very theory that suggests that time travel may be possible in the first place. Goldberg asserts that, to make your time travel fiction scientifically sound, you need to stay within the one universe you’ve been given.

2) You can’t visit any time before your time machine was built.

In Einstein’s universe, time and space are closely related to each other. Therefore, time travel could be imagined as a trip through a tunnel in space – a tunnel that has a way in and a way out – an entrance/exit at each end. The rule that follows, then, is that both ends of the tunnel need to be in a time when time travel exists.

If you think about it, the application of this rule would help explain why, if time travel is indeed possible and will one day be perfected, we don’t receive visitors who are time traveling back to us from the future. Since time travel hasn’t been “invented” yet, time travelers from the future cannot come back to our time. (I suppose it follows that, once it is invented, we can expect to meet people from the future as they come back to take a tour of our time.)

3) You can’t kill your own grandfather.

This rule concerns the consistency of history. It looks at something known as the “grandfather paradox,” which goes something like this:

Imagine you possess a time machine. You go back in time and decide to kill your own grandfather. Now what? Well, if you kill your grandfather, you will never be born, and if you are never born, you won’t exist to come back and kill your grandfather, which means you will be born.

In the mid-1980s, physicist Igor Novikov used quantum mechanics to develop the “self-consistency theorem,” which demonstrates that there is no actual possibility of changing history with a time machine. The events of history cannot be altered, according to Novikov. Even if you went back in time and tried to kill your own grandfather, you wouldn’t be able to, because the events of history are fixed.

4) You don’t have nearly as much free will as you think you do.

Novikov’s “self consistency theorem” can be frustrating and difficult to accept. Don’t we all like to believe that we control our own destinies?

But what about the destiny of an inanimate object? Instead of your grandfather’s life, what if we were dealing with the movement of pool balls? Imagine a time machine set up so that a pool ball shot into the time machine came out a second earlier. Shouldn’t it be possible to aim a shot so that, when the pool ball emerged from the time machine a second in the past, it blocked the original shot that sent the pool ball into the machine in the first place?

Kip Thorne and his students examined the paradox of the “impossible pool shot” and came up with a compromise. They propose that, no matter how hard you tried to line up the shot perfectly, there would be some unplanned angle to the original shot. The ball would then come out of the machine at a slightly askew angle, thereby clipping the first ball just enough to send it into the time machine slightly askew, and the whole thing would continue in an endless perfect loop.

In other words, a physicist would argue that once you’ve seen your destiny by traveling to the future, there is nothing you can do to change it.

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What do you think of Goldberg’s four rules of time travel? Do you think time travel fiction should follow rules? Do you believe that writers of science fiction should keep the “science” in mind while plotting? Please share your thoughts in the comments!

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Julie Eshbaugh is represented by Natalie Fischer of the Bradford Literary Agency. You can read her blog here and find her on Twitter here.

32 Responses to “Fictional Time Travel That Won’t Make a Physicist Cringe”

I do believe that SF should try to follow the rules as much as possible, within the context of a good story. If the rules have to bend for that, so be it. I would have more respect for an author who didn’t have to bend the rules too much to make his story work, though.

That said, The End of Eternity and The Proteus Operation are two of my favorite time travel novels and they both violate some of these rules. The Eternity organization exists outside of ‘Reality’, although it does have a limit on how far back in time they can go. The Proteus Operation also has limits, but these are solely based on available power. They avoid the Grandfather Paradox by having the only way to travel back in time be to the past of a different universe, which violates the first rule you mentioned. Which really isn’t so much a rule as an unprovable supposition.
My favorite time travel stories are not forward and back but lateral, alternate history stories like H. Beam Piper’s Paratime and Lord Kalvan books.

I write YA science fiction & paranormal. My sci fi MS had a lot of science information in it. I did quite a lot of research before I began to write it to ensure that the science portions were correct. I think that it’s important to have as much factual info correct as possible. That way when you want to embellish and stretch the truth a little, it goes off without a hitch.

Interesting stuff! For a really fun read on playing with time, try ‘Einstein’s Dreams.’ It’s a beautifully written collection of essays by a physicist (seriously jealous of this guy–beautiful writing and a scientific genius? Wow.) It’s less strict science and more fun/introspective speculation, but it’s a great exercise in “what if”–what if time the way I understand it worked differently?

I used time travel in my book The Noble Pirates, and I read Goldberg’s article before tackling the issue. I wanted the time travel element to be as close to the scientific truth as possible. I DO think it’s important for writers to stick close to the truth, even when writing paranormal or fantasy. It shows that the writer cares to research, and it also lends more credibility to the story itself.

Hi Rima! I agree with you about the importance of credibility. I can’t stand it when characters get into a jam and some handy-dandy magic that follows no rules at all saves them. There’s no sense of a solution if the “magic” has no restrictions. Thanks for your comment!

I definitely think following as many rules as possible when dealing with time travel is so, so helpful. Time travel is tricky, and even when you manage to get the loopy timeline worked out, there’s still the possibility of readers questioning it. “But if they can do x, why can’t they do y?” Following an established set of rules helps with that confusion.

I always love it when SF turns out to follow the rules, because with all the other researching that usually has to be done for a novel, putting science together like that is impressive. But as someone who doesn’t really understand science and is happy and willing to suspend his disbelief, I’m also of the opinion that if you write your science believably enough, I will enjoy it just as much as the real stuff.

Hey Mandy! I think of PRADA & PREJUDICE as more of an example of time travel fantasy than time travel SF. If only those Prada heels were scientifically possible! I’d be dancing alongside Dukes and Lords right now. 😉

i’m not a physicist…but as someone who spends most of my life thinking about and doing science:

un-realisism in science fiction doesn’t bother me much. it is fiction, it’s allowed to play with the rules (or make new ones up). what *does* bother me is when science fiction is wrong. wrong, as in, there is knowledge that directly contradicts a plot point/plot device. this is especially galling when the fiction the book/movie/whatever is using is somehow already (or becomes because of the fiction) part of the public’s misconception of the science. theoretical things (like time travel) don’t bother me as much because they are just that–theoretical, and so there is no empirical evidence to support or contradict any imagined use of them. [A/N: i concede that uses of theoretical science like time travel may not bother me as much because that’s not my area of interest/study] case in point: in a recent movie, the myth that we only use a small fraction of our brains–and think how much more we could do if we could use all that brainpower!–was perpetuated. this is wrong. empirically wrong. i think this kind of thing annoys me so much because it’s passed off as science, and the public (reading or watching) may not actually know better (i think ivory towers of academe are embarassingly bad about engaging with and informing the public). so i guess, when it’s clear that the science fiction device IS fiction, i’m a-okay with it. if it presents science incorrectly–i think it can be cringe-worthy.

Hey Kate, thanks for that great comment! I used to work for a criminal defense attorney, and I would frequently get very upset when our criminal justice system was so horrendously misrepresented in books or films, simply because it served the plot. I agree that blatant lack of accuracy with regard to known truth is cringe-worthy. 🙂

This was such an interesting read! I’ll save the Goldberg article for this afternoon, when I have some more time, but congrats for this.

And yes, not travelling to the past is a little sad… mostly because it makes me think of that Futurama episode where Fry has sex with his grandmother, which in turns makes him his own grandfather.
Twisted, but far more entertaining than killing grandpa (which he does anyway, by leaving him in a atomic bomb test field).

Eeeeeer yes, I’m an absolute Geek. Which is why I won’t talk about LOST here. You got me curious to read The Time Traveller’s Wife, too. Good article, Julie!

In my current working project, traveling through wormholes gets you where you want to go instantaneously, from your perspective… but if certain conditions aren’t met, you might end up traveling two months into the past, or seven years into the future. So, time travel isn’t so much a plot device as it is a limitation of the setting.

I followed all of the rules mentioned here, at least partly because I read Kip Thorne’s book while I was in the planning stages, and it worked out really well. My one regret is that I didn’t have the chance to drop the phrase “Novikov consistency principle,” which just sounds cool.

Well Julie, I am a physicist, so this article really caught my attention.

So now speaking as a physicist, most SF really does not bother me, even when there is no science that would make some ideas even seem remotely possible.

Take lightsabers for instance. There are a couple ways I could think of that would allow you to concentrate that much light in a sword form, but trust me, there is no way that would ever look like what we see in Star Wars. But do I care? Nope.

The key is to tell a neat story, and use the technology as a tool to make your world unique. Just don’t try to explain exactly how you think your technology works if you don’t really understand the science. That is what makes me angry at SF. Point and case would be the movie 2012 where they claim that neutrinos super-heated the earth’s core and went on at length about it. That took me so out of the story I didn’t even appreciate the special effects.

The bottom line is that you shouldn’t worry about whether or not your technology would work, it just matters that it works in your story.

Oh, and I’ll have to disagree with Goldberg on the “many worlds” rule. That is an interpretation of QM that is equally valid with the Copenhagen one (I do agree with him that he is being overly dogmatic). The equations of QM work just as well with the many worlds version as with Copenhagen.

QM is really just a lot of math that models behavior. The interpretation is what we think causes the math. For instance a falling ball is modeled with a simple equation. If I use the same math as you to predict how fast a ball will fall as you, but you say that gravity causes it to fall, and I say the ball is just in love with the earth, we both get the right answer…

Many of my friends like the film Primer for the way it portrays time-travel. This is an interesting dilemma, I think above all else that if you are violating a rule it should be intentionally with full knowledge of the rule rather than a result of ignorance. I know several people who have studied high levels of math and physics who really enjoy science fiction, even if it is somewhat inaccurate or improbable, but only if it has good reason to be so.

For sort-of an example, these friends love the film How to Train your Dragon despite the fact that dragons are improbable (something about insects being the only creatures that don’t have to give up a set of appendages to gain wings) because the actual mechanics of the flight is pretty spot on (the wind-tunnel sequence was a particular favorite of theirs.)

LOL shows what I know about wings and appendages. I never knew that was one of the factors that made dragons improbable. But HOW TO TRAIN YOUR DRAGON made me wish they were real. 🙂 Thanks for your comment, Caitlin.

I just discovered this by chance, and wanted to thank you for the shout-out. As it happens, I addressed some of your readers ideas in my blog when the slate article first came out (and in particular Primer and the Many Worlds Interpretation, which were biggies). Link is here:

Hey Dave,
I was really hoping you would find this post! I’m glad you were happy with it, and thanks for the link and the info about the book. I will surely check them out and I’m sure the blog’s readers will, too. Thanks so much for stopping by and commenting! 🙂